PURPOSE: To make it possible to detect definitely the reversal motion of a switchgear which drives forwardly and reversely with an output from a frequency conversion circuit device and hence eliminate the need for a large volume of frequency conversion circuit device which exceeds a desired value.
CONSTITUTION: A waveform detection device means 9 is installed to an inverter current which leads to an electric motor 2a from a frequency conversion circuit device 5 while an overcurrent detection means 10 and a higher harmonic detection means 11 are installed to the waveform detection means 9. When an overcurrent flows in an inverter current, it is so arranged that a detection signal may be output to a switch drive control circuit 6 if there is a higher harmonic. The switch drive control device 6 gives a shutdown command to the frequency conversion circuit device 5 based on at least one detection signal received and outputs a braking command to a braking device 2b, thereby halting the movement of a shutter curtain in emergency mode.
YAMAMOTO YASUSHI
KOSHII NORIHIRO
